Immunosuppressive networks in the tumour environment and their effect in dendritic cells

The failure of the immune system to provide protection against tumour cells is an important immunological problem. It is now evident that inadequate function of the host immune system is one of the main mechanisms by which tumours escape from immune control, as well as an important factor that limits the success of cancer immunotherapy. In recent years, it has become increasingly clear that defects in dendritic cells have a crucial role in non-responsiveness to tumours. This article focuses on the functional consequences and recently described mechanisms of the dendritic-cell defects in cancer.     

NK cells and apoptosis

Natural killer (NK) cells are a cell of the innate immune system that play an important role in the early response to viral infections and tumours. Natural killer cells are cytolytic, and secrete cytokines that influence the developing antigen-specific immune response. In the present article the NK cell surface molecules regulating effector function, the NK cell effector mechanisms involved in apoptosis, and the role of NK cell effector mechanisms in immune responses are reviewed.     

Challenges facing adjuvants for cancer immunotherapy

An adjuvant is defined as a product that increases or modulates the immune response against an antigen (Ag). Based on this general definition many authors have postulated that the ideal adjuvant should increase the potency of the immune response, while being non-toxic and safe. Although dozens of different adjuvants have been shown to be effective in preclinical and clinical studies, only aluminium-based salts (Alum) and squalene-oil-water emulsion (MF59) have been approved for human use. However, for the development of therapeutic vaccines to treat cancer patients, the prerequisites for an ideal cancer adjuvant differ from conventional adjuvants for many reasons. First, the patients that will receive the vaccines are immuno-compromised because of, for example, impaired mechanisms of antigen presentation, non-responsiveness of activated T cells and enhanced inhibition of self-reactivity by regulatory T cells. Second, the tumour Ag are usually self-derived and are, therefore, poorly immunogenic. Third, tumours develop escape mechanisms to avoid the immune system, such as tumour editing, low or non-expression of MHC class I molecules or secretion of suppressive cytokines. Thus, adjuvants for cancer vaccines need to be more potent than for prophylactic vaccines and consequently may be more toxic and may even induce autoimmune reactions. In summary, the ideal cancer adjuvant should rescue and increase the immune response against tumours in immuno-compromised patients, with acceptable profiles of toxicity and safety. The present review discusses the role of cancer adjuvants at the different phases of the generation of antitumour immunity following vaccination.     

TGFbeta is responsible for skin tumour infiltration by macrophages enabling the tumours to escape immune destruction

Infiltration of skin tumours by macrophages is an important step in tumour progression, although the mechanisms of macrophage recruitment to the tumour mass and the subsequent effects on tumour growth are poorly understood. Transfecting a murine regressing skin tumour with the gene for transforming growth factor (TGF)beta enabled the tumours to grow progressively in vivo thus allowing us to study the role of this cytokine in tumour growth. Flow cytometry was used to show that TGFbeta-mediated tumour progression was accompanied by an increase in tumour-associated macrophages (TAM) and a decrease in tumour-infiltrating dendritic cells (DCs). TAM in TGFbeta-secreting tumours expressed lower levels of major histocompatibility complex II and CD86 compared to DC in control tumours and had a high phagocytic capacity as measured by uptake of latex beads in vivo. Indeed, TGFbeta was directly responsible not only for the enhanced macrophage phagocytosis but also altering the ratio of antigen-presenting cells to favour macrophages over DC. Our results demonstrate that TGFbeta recruitment and retention of macrophages at the tumour site enable effective tumour evasion of the host immune system and reinforces the need to target TGFbeta in human cancer immunotherapy trials.     

The p53 network: cellular and systemic DNA damage responses in aging and cancer

Genome instability contributes to cancer development and accelerates age-related pathologies as evidenced by a variety of congenital cancer susceptibility and progeroid syndromes that are caused by defects in genome maintenance mechanisms. DNA damage response (DDR) pathways that are mediated through the tumor suppressor p53 play an important role in the cell-intrinsic responses to genome instability, including a transient cell cycle arrest, senescence and apoptosis. Both senescence and apoptosis are powerful tumor-suppressive pathways preventing the uncontrolled proliferation of transformed cells. However, both pathways can potentially deplete stem and progenitor cell pools, thus promoting tissue degeneration and organ failure, which are both hallmarks of aging. p53 signaling is also involved in mediating non-cell-autonomous interactions with the innate immune system and in the systemic adjustments during the aging process. The network of p53 target genes thus functions as an important regulator of cancer prevention and aging.     

Blocking retinoic acid receptor-α enhances the efficacy of a dendritic cell vaccine against tumours by suppressing the induction of regulatory T cells

The immune system has evolved regulatory mechanisms to control immune responses to self-antigens. Regulatory T (Treg) cells play a pivotal role in maintaining immune tolerance, but tumour growth is associated with local immunosuppression, which can subvert effector immune responses. Indeed, the induction and recruitment of Treg cells by tumours is a major barrier in the development of effective immunotherapeutics and vaccines against cancer. Retinoic acid (RA) has been shown to promote conversion of naïve T cells into Treg cells. This study addresses the hypothesis that blocking RA receptor alpha (RARα) may enhance the efficacy of a tumour vaccine by inhibiting the induction of Treg cells. We found that RA significantly enhanced TGF-β-induced expression of Foxp3 on naïve and committed T cells in vitro and that this was blocked by an antagonist of RARα (RARi). In addition, RARi significantly suppressed TGF-β and IL-10 and enhanced IL-12 production by dendritic cells (DC) in response to killed tumour cells or TLR agonists. Furthermore, RARi augmented the efficacy of an antigen-pulsed and TLR-activated DC vaccine, significantly attenuating growth of B16 tumours in vivo and enhancing survival of mice. This protective effect was associated with significant reduction in tumour-infiltrating FoxP3⁺ and IL-10⁺ Treg cells and a corresponding increase in tumour-infiltrating CD4⁺ and CD8⁺ T cells that secreted IFN-γ. Our findings demonstrate that RARα is an important target for the development of effective anti-tumour immunotherapeutics and for improving the efficacy of cancer vaccines.     

Immunological responses can have both pro- and antitumour effects: implications for immunotherapy

Immune responses influence the development and progression of a malignancy. The tumour can also manipulate the immune system to its own ends, often resulting in an ineffective or transient antitumour response. An appreciation of the complexity of these host-tumour interactions is therefore important for the development of more-effective cancer therapies. This article highlights some prominent mechanisms whereby tumours escape recognition and destruction by the host immune system, thus facilitating disease progression. One important consequence of tumour escape is that an antitumour immune response may unintentionally lead to the outgrowth of less immunogenic or more apoptosis-resistant tumour escape variants, which possess enhanced tumourigenic potential. Insights into the molecular mechanisms of cancer evasion and the complexity of the ever-changing interactions between host and tumour will enable a more rational design of antitumour therapies and may help not only explain disease recurrence, but also identify potential targets for therapeutic interventions. This article also offers a brief review of preclinical animal models, which are essential tools in the study of tumour immunology and cancer biology, particularly those that recapitulate the chronic nature of host-tumour interactions and help guide the development and testing of new therapies.     

Tumour exosomes inhibit binding of tumour-reactive antibodies to tumour cells and reduce ADCC

In order to grow within an immunocompetent host, tumour cells have evolved various strategies to cope with the host’s immune system. These strategies include the downregulation of surface molecules and the secretion of immunosuppressive factors like IL-10 and PGE2 that impair the maturation of immune effector cells, among other mechanisms. Recently, tumour exosomes (TEX) have also been implicated in tumour-induced immune suppression as it has been shown that TEX can induce apoptosis in T lymphocytes. In this study, we extend our knowledge about immunosuppressive features of these microvesicles in that we show that TEX efficiently bind and sequester tumour-reactive antibodies and dramatically reduce their binding to tumour cells. Moreover, we demonstrate that this antibody sequestration reduces the antibody-dependent cytotoxicity by immune effector cells, which is among the most important anti-tumour reactions of the immune system and a significant activity of therapeutic antibodies. Taken together, these data point to the fact that tumour-derived exosomes interfere with the tumour-specific function of immune cells and constitute an additional mechanism how tumours escape from immune surveillance.     

Characterization of the relationship between FLI1 and immune infiltrate level in tumour immune microenvironment for breast cancer

Breast cancer is the most common cancer and the leading cause of cancer death among women in the world. Tumour-infiltrating lymphocytes were defined as the white blood cells left in the vasculature and localized in tumours. Recently, tumour-infiltrating lymphocytes were found to be associated with good prognosis and response to immunotherapy in tumours. In this study, to examine the influence of FLI1 in immune system in breast cancer, we interrogated the relationship between the FLI1 expression levels with infiltration levels of 28 immune cell types. By splitting the breast cancer samples into high and low expression FLI1 subtypes, we found that the high expression FLI1 subtype was enriched in many immune cell types, and the up-regulated differentially expressed genes between them were enriched in immune system processes, immune-related KEGG pathways and biological processes. In addition, many important immune-related features were found to be positively correlated with the FLI1 expression level. Furthermore, we found that the FLI1 was correlated with the immune-related genes. Our findings may provide useful help for recognizing the relationship between tumour immune microenvironment and FLI1, and may unravel clinical outcomes and immunotherapy utility for FLI1 in breast cancer.     

The resolution of inflammation and cancer

Inflammation has long been thought to contribute to the development of cancer; however there is also clear evidence that the immune system can recognize and eliminate cancer cells. Current research suggests that cancer-associated inflammation has a dual role in tumor progression; inflammatory mediators promote the malignant activity of cancer cells by acting as growth factors and also stimulate angiogenesis, however, cancer-associated inflammation is also linked with immune-suppression that allows cancer cells to evade detection by the immune system. In this review we will discuss the dual role of inflammation in cancer and how endogenous anti-inflammatory mechanisms may equally be important in carcinogenesis.     

Cancer immunotherapy: the past,the present and the future

One of the most controversial issues in immunology for over a century has been whether an effective immune response can be elicited against malignant tumours. Whether the immunology community has believed cancer immunotherapy is feasible or impossible has been largely determined by the prevailing immunological paradigms at that time. In fact, during the last 110 years it is possible to trace at least five dramatic fluctuations in attitude towards cancer immunotherapy. It now appears, however, that overwhelming evidence is available to support the view that both the innate and adaptive immune responses can recognize and eliminate tumours. On the other hand, it remains to be seen if these immune responses can be harnessed to control cancer as, at the time of diagnosis, many tumours have already been immunoselected to be highly resistant to immune elimination. Based on these observations it is argued that immunotherapy approaches, other than the generation of tumour-specific cytotoxic T lymphocytes, must be explored. Alternative strategies include recruiting tumouricidal myeloid cells into tumours, generating antiangiogenic immune responses and directing innate immunity to hypoxia-induced ligands on tumour cells.     

Combining oncolytic virotherapy and tumour vaccination

The interactions between the immune system, a malignant tumour and an oncolytic virus are complex and poorly understood. For oncolytic viruses to become successful therapeutics we need to better understand these interactions and identify strategies to take advantage of defects in the innate immune response within tumours and avoid cellular anti-viral responses while capitalizing on anti-tumoural immunity. In this review we will discuss the evidence for the induction of tumour-specific immune responses by oncolytic viruses as well as by cancer vaccines. We will then describe some of the barriers to successful cancer immunotherapy, and finally we will outline a strategy for enhancing anti-tumoural immunity while reducing anti-viral immunity by combining tumour vaccination with oncolytic viral therapy.     

Dendritic cells as therapeutic vaccines against cancer

Mouse studies have shown that the immune system can reject tumours, and the identification of tumour antigens that can be recognized by human T cells has facilitated the development of immunotherapy protocols. Vaccines against cancer aim to induce tumour-specific effector T cells that can reduce the tumour mass, as well as tumour-specific memory T cells that can control tumour relapse. Owing to their capacity to regulate T-cell immunity, dendritic cells are increasingly used as adjuvants for vaccination, and the immunogenicity of antigens delivered by dendritic cells has now been shown in patients with cancer. A better understanding of how dendritic cells regulate immune responses will allow us to better exploit these cells to induce effective antitumour immunity.     

Oxygen as a regulator of cellular phenotypes in pregnancy and cancer

Cellular phenotype is determined by genetic and microenvironmental factors. There is evidence that tissue oxygenation status is one of the microenvironmental factors regulating cellular behaviour. Both normal and pathological processes such as blastocyst implantation in the uterus, placentation, and rapidly growing tumours occur under conditions characterized by relatively low oxygen levels. In this review, we address the effects of low oxygen concentrations on the phenotype of trophoblast and cancer cells. We provide evidence that oxygenation levels play an important role in the regulation of normal and pathological cellular invasiveness as it occurs during trophoblast invasion of the uterus and in tumour progression and metastasis, drug resistance in cancer, and antitumour activity of natural killer cells of the immune system.     

EMT: A mechanism for escape from EGFR-targeted therapy in lung cancer

Epithelial mesenchymal transition (EMT) is a reversible developmental genetic programme of transdifferentiation of polarised epithelial cells to mesenchymal cells. In cancer, EMT is an important factor of tumour cell plasticity and has received increasing attention for its role in the resistance to conventional and targeted therapies. In this paper we provide an overview of EMT in human malignancies, and discuss contribution of EMT to the development of the resistance to Epidermal Growth Factor Receptor (EGFR)-targeted therapies in non-small cell lung cancer (NSCLC). Patients with the tumours bearing specific mutations in EGFR have a good clinical response to selective EGFR inhibitors, but the resistance inevitably develops. Several mechanisms responsible for the resistance include secondary mutations in the EGFR gene, genetic or non-mutational activation of alternative survival pathways, transdifferentiation of NSCLC to the small cell lung cancer histotype, or formation of resistant tumours with mesenchymal characteristics. Mechanistically, application of an EGFR inhibitor does not kill all cancer cells; some cells survive the exposure to a drug, and undergo genetic evolution towards resistance. Here, we present a theory that these quiescent or slow-proliferating drug-tolerant cell populations, or so-called “persisters”, are generated via EMT pathways. We review the EMT-activated mechanisms of cell survival in NSCLC, which include activation of ABC transporters and EMT-associated receptor tyrosine kinase AXL, immune evasion, and epigenetic reprogramming. We propose that therapeutic inhibition of these pathways would eliminate pools of persister cells and prevent or delay cancer recurrence when applied in combination with the agents targeting EGFR.     

Control of aromatase activity in breast tumours: the role of the immune system

Cytokines such as interleukin-6 (IL-6) and tumour necrosis factor alpha (TNF), have been identified as important regulators of aromatase activity in fibroblasts derived from normal and malignant breast tissues, and may play an important role in controlling aromatase activity in breast tumours. The major source of such cytokines within breast tumours remains to be established but macrophages and lymphocytes, which can infiltrate tumours, have been identified as a potential source of aromatase stimulatory cytokines. To obtain further insight into the possible role played by the immune system in cancer development, and in particular its ability to regulate aromatase activity via cytokine production, we have obtained peripheral blood monocytes and lymphocytes from an immunosuppressed kidney transplant recipient, receiving cyclosporin A therapy, and a woman with breast cancer. Monocytes and lymphocytes were stimulated with lipopolysaccharide (LPS), and the conditioned medium (CM) collected from these cells was tested for its ability to stimulate aromatase activity in fibroblasts derived from normal breast tissue from a woman undergoing lumpectomy for the removal of a breast tumour. The white blood cell count was lower for the immunosuppressed patient, mainly because of the reduction in the number of monocytes and lymphocytes. The ability of CM from the monocytes and lymphocytes of the immunosuppressed patient to stimulate aromatase activity was significantly reduced (68% and 82% for monocytes and lymphocytes, respectively) compared with that of CM from the cells of the woman with breast cancer. It is possible, therefore, that immunosuppression, which has been found to be associated with a reduction in the incidence of de novo breast cancer in kidney transplant recipients, may exert its effect by inhibiting cytokine production by the cells of the immune system and thus oestrogen synthesis. In contrast to the stimulatory effects that TNF has on aromatase activity in breast fibroblasts, in MCF-7 breast cancer cells, which possess low aromatase activity, it reduced activity. However, the extent of inhibition of aromatase activity in these epithelial cells was much lower than the marked stimulation which it can induce in breast fibroblasts.     

综述:髓系衍生的抑制性细胞与肿瘤免疫耐受关系的研究进展

髓系衍生的抑制性细胞(myeloid-derived suppressor cells,MDSCs),是在肿瘤等病理因素的作用下髓系细胞发生分化障碍所产生的不同阶段髓系祖细胞的集合,具有广谱而强大的免疫抑制功能,是免疫系统的重要负性调节组件之一.研究表明:肿瘤微环境中的多种细胞因子或生长因子可通过激活相应的信号通路促进MDSCs扩增及活化,MDSCs进而通过多种机制抑制包括T细胞在内的多种免疫细胞的功能而促进肿瘤个体免疫耐受的发生.临床研究表明:肿瘤患者体内MDSCs的水平与肿瘤临床病程进展密切相关,基于MDSCs的免疫治疗也有望成为肿瘤免疫治疗的新策略.本文主要介绍了肿瘤中MDSCs的表型鉴定、扩增及活化机制、发挥免疫抑制作用的途径及机制、肿瘤中MDSCs的临床意义以及本领域需要解决的问题,以期对MDSCs在肿瘤免疫耐受中的作用进展提供参考.     

Tumour eradication and induction of memory against murine mesothelioma by combined immunotherapy

Numerous immunotherapy treatments for cancer are undergoing clinical trials or are already approved for use. One particular area of interest is targeting mechanisms of immune tolerance. Using a murine model of mesothelioma, we investigated the roles of regulatory T-cells, intratumoural transforming growth factor (TGF)-β and the negative regulator molecule cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) in immune tolerance to tumours. It was found that treatments targeting a single negative regulator molecule mechanism were not as effective against tumours as targeting multiple mechanisms simultaneously. Most importantly, it was found that a combined triple treatment of anti-CD25 monoclonal antibody (mAb), anti-CTLA-4 mAb and TGF-β soluble receptor resulted in long-term clearance of tumours and memory against tumour rechallenge. These data suggest that clinical application of immunotherapies against tumours may be improved by simultaneously targeting multiple mechanisms of immune suppression.     

Colon cancer metastasis in mouse liver is not affected by hypercoagulability due to Factor V Leiden mutation

Clinical trials have shown life-prolonging effects of antithrombotics in cancer patients, but the molecular mechanisms remain unknown due to the multitude of their effects. We investigated in a mouse model whether one of the targets of antithrombotic therapy, fibrin deposition, stimulates tumour development. Fibrin may provide either protection of cancer cells in the circulation against mechanical stress and the immune system, or form a matrix for tumours and/or angiogenesis in tumours to develop. Mice homozygous for Factor V Leiden (FVL), a mutation in one of the coagulation factors that facilitates fibrin formation, were used to investigate whether hypercoagulability affects tumour development in an experimental metastasis model. Liver metastases of colon cancer were induced in mice with the FVL mutation and wild-type littermates. At day 21, number and size of tumours at the liver surface, fibrin/fibrinogen distribution, vessel density and the presence of newly formed vessels in tumours were analysed. Number and size of tumours did not differ between mice with and without the FVL mutation. Fibrin/fibrinogen was found in the cytoplasm of hepatocytes and cancer cells, in blood vessels in liver and tumour tissue and diffusely distributed outside vessels in tumours, indicating leaky vessels. Vessel density and angiogenesis varied widely between tumours, but a pre-dominance for vessel-rich or vessel-poor tumours or vessel formation could not be found in either genotype. In conclusion, the FVL mutation has no effect on the development of secondary tumours of colon cancer in livers of mice. Fibrin deposition and thus inhibition of fibrin formation by anticoagulants do not seem to affect tumour development in this model.     

Relationship between immune state and tumor growth rate in rats bearing progressive and non-progressive mammary tumors

 Impaired immune responses occur frequently in cancer patients or in tumor-bearing animals, but the mechanisms of the tumor-induced immune defects remain poorly understood. The aim of the present study was to determine the relevance of the immune system in the control of tumor growth. We have developed a model of progressive and non-progressive mammary tumor, chemically induced in female Wistar rats. In this model we evaluated the development of an immune response after immunization of rats bearing progressive and non-progressive tumors with a non-related antigen, such as sheep red blood cells. We also studied the activation state of peritoneal macrophages from animals bearing tumors by evaluating the production of free radicals. Our findings indicated that the cell-mediated immunity in rats bearing progressive tumors fails to respond to heterologous antigen in vivo, as demonstrated by a negative delayed-type hypersensitivity reaction, and is accompanied by minor nitric oxide production by peritoneal exudate cells as well as a lower capacity for macrophage activation. The study of non-progressive tumor-bearing rats indicated that the cell-mediated immune response was intact and an activated state of macrophages was found in vivo. The results described in this paper should be taken into account when therapies based on cancer vaccines are chosen for the treatment of cancer. Received: 26 February 1998 / 9 April 1998